Lecture 6: Defining your own Classes

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Lecture 6 Defining your own Classes
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Last time

• We finished up conditionals

if (hoop.contains(point)) score score 2
if (hoop.contains(point)) baskets baskets
1 else misses misses 1
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Last time

• We learned about booleans
• We learned how to use these to make onMouseDrag()
  a bit more clean and efficient

boolean carryingBall public void
onMousePress(Location point) carryingBall
ball.contains(point) public void
onMouseDrag(Location point) if (carryingBall)
//do something
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Conditional assignment to booleans

• How can you make this code simpler?

if (ball.contains(point)) carryingBall
true else carryingBall false
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Conditional assignment to booleans

• How can you make this code simpler?
• Like this (we just saw it)

if (ball.contains(point)) carryingBall
true else carryingBall false
carryingBall ball.contains(point)
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Conditional assignment to booleans

• How about this code?
• Theres a subtle difference

if (ball.contains(point)) missedBall
false else missedBall true
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Conditional assignment to booleans

• How about this code?
• Theres a subtle difference
• If you see true false reversed in an if/else,
  just use not (!)

if (ball.contains(point)) missedBall
false else missedBall true
missedBall !ball.contains(point)
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Some notes on style

• 2 styles tyically used for braces

public void begin() if (condition) //do
something else //do something else
public void begin() if (condition) //do
something else //do something else
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Some notes on style

• Weve seen 4 kinds of names so far
• Class names
• Starts with capital
• Camel-case
• Method names
• Starts with lowercase
• Camel-case

FilledRect, NoClicking, FramedOval
onMouseClick(), begin(), onMouseDrag()
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Some notes on style

• Variable names
• Same as method names
• Distinguishable by parentheses after method call
  or declaration
• Constant names
• All caps
• Words separated by underscores

someInt, myVariable, numberOfHits, myRectangle
SWATCH_LOCATION, BOX_WIDTH, BOX_LEFT, BOX_RIGHT
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Some notes on style

• Comments
• Typically go above or to the side of what youre
  talking about

/ Oval to represent the basketball.
/ FilledOval ball
FilledOval ball //Oval to represent the
basketball
// This method draws a sign and initializes
instance variables public void begin()
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Basketball

• Were going to keep using our basketball game
• This time, though, Ive made it more realistic
• The basketball has color and lines now!
• How would you write this?
• How would you move the ball?

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Basketball

• You could use 6 instance variables
• 1 FilledOval
• 1 FramedOval
• 2 Lines
• 2 Arcs (these are new)
• Youd have to construct all 6 in begin()
• Youd have to move all 6 in onMouseDrag()
• Any ideas for a better way?

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Basketball
// program that displays how many times the mouse
has been clicked public class Basketball extends
WindowController private FramedOval
hoop // the oval that represent the hoop
private BBall ball // this is the ball
public void begin() ball new
BBall( BALLX, BALLY, BALLSIZE, canvas)
// Move the ball public void
onMouseDrag(Location point) if (
ball.contains(lastMouse) )
ball.move( point.getX() - lastMouse.getX(),
point.getY() - lastMouse.getY()
) lastMouse point
// increment the counter and
update the text public void
onMouseRelease(Location point) if
( hoop.contains(point) ball.contains(lastMouse)
) score score 2
display.setText("Your score is " score
) ball.moveTo(BALLX,B
ALLY)

• Look at the code for the new Basketball program
• at left is the code w/some things omitted)
• Just the code for the ball
• Instead of 6 instance variables, it has just one!
• Variable is of type BBall

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Whats BBall?

• Its a basketball
• Is that part of objectdraw?
• No!
• We wrote it ourselves
• Somehow, in Java, we can make new classes of
  objects
• These can be designed to fit the program were
  writing.

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Basics of classes

• Youve already been writing classes
• And, you know the basics of what a class needs
• Remember the robot from the first day of class?
• A class describes a part of the robot
• Methods describe things you can tell it to do

public class Name constant and variable
declarations methods
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WindowController

• How do our classes so far fit the robot part
  model?
• Youve been writing classes that respond to mouse
  actions
• Mouse actions were the methods
• You told your classes how to behave when they
  received mouse clicks, drags, etc.
• Who calls these methods?
• The system knows when you extend WindowController
  that it should call them
• It calls them when the user does things with the
  mouse.

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BBall

• BBall isnt going to extend WindowController
• With BBall, were not describing mouse events
• Were writing methods to describe things we want
  to tell our basketball
• e.g. move moveTo
• Not onMouseClick, onMouseDrag, etc

public class BBall Location position
//instance variable public void move(int dx,
int dy) //method telling how to move a BBall

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Whats in a BBall?

• Classes need
• Whatever instance variables are necessary to
  describe the parts of an object of the class
• BBall is made up of ovals, arcs, lines so its
  instance vars will be circles, arcs, and lines.
• A special method called a constructor
• Youve used these (with new), you just havent
  written one
• Constructor sets up the parts of the object
• Builds the instance vars
• Methods
• Statements describing how to do what we want the
  object to do

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Whats in a BBall?

• Lets look at the BBall code

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Constructors

• These perform actions that must be undertaken
  when an object is created
• Similar to begin()
• Provide initial values of instance variables
• Create graphic objects
• To declare a constructor
• Name is the same as that of the class it belongs
  to
• No void before methods name here
• Constructor needs no type -- it builds an object
  of the classs type!

public ClassName(params )
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Constructors

• Heres the BBall constructor declaration
• Has parameters for
• starting location of upper left part of ball
• Size of basketball
• The canvas
• The type DrawingWindow is new
• This is the type of the canvas weve been using
• Need canvas for all the graphical object
  constructors
• FilledRect, FramedRect, etc

public BBall(double left, double top, double
size, DrawingCanvas aCanvas)
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Constructors

• Heres the BBall constructor declaration
• To call this, you would just use new
• Exactly the way weve been doing it with the
  graphics objects

public BBall(double left, double top, double
size, DrawingCanvas aCanvas)
new BBall(50, 50, 60, canvas)
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Mutator Methods
public void methodName(ParameterType parameter,
...) . . .

• First keyword in a method header is public or
  private
• Determines visibility
• public methods can be used by other classes
• private methods can only be used by their own
  class
• You couldnt call setColor() on the FilledRect if
  it wasnt public
• If it was private, the compiler wouldnt let you
  run this
• Youre calling FilledRects setColor() from
  NoClicking

public class NoClicking public void begin()
myRect new FramedRect(100, 100, 200, 200,
canvas) myRect.setColor(Color.yellow)
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Visibility
public void methodName(ParameterType parameter,
...) . . .

• Your methods are usually public (so far)
• In contrast, we always make instance variables
  private
• This is so that theyre not accessible to other
  classes
• Cant alter BBalls internals from outside BBall
• Having private instance variables guarantees that
  the only way someone can change your object is
  through its methods, in the way you specify

private Text textBox
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Mutator Methods

• Second keyword in methods header is void
• Indicates that this is a mutator method
• That it just does a task rather than returning a
  value
• Third part of method header is the methods name
• This is your choice!
• Weve been using special names so far
  (onMouseClick, etc)
• You can make your own classes have methods called
  anything

public void methodName(ParameterType parameter,
...) . . .
public void methodName(ParameterType parameter,
...) . . .
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Mutator Methods

• Last thing in a method header are formal
  parameters
• This is a specification of what you have to call
  the method with
• So far weve seen methods that require one piece
  of info
• The Location, point, is determined by the system
  based on where the mouse is, and then the system
  calls onMouseClick() and passes point in
• Formal parameters are very similar to variable
  declarations

public void methodName(ParameterType paramName,
...) . . .
public void onMouseClick(Location point) . . .
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Mutator Methods

• You can make method headers that require more
  than one piece of information
• Just use more than one formal parameter
• You can then call these methods with
  appropriately typed actual parameters
• We declare that move takes 2 doubles, xOffset and
  yOffset
• We can then call it with 2 doubles (55 and 70)

public void move(double xOffset, double yOffset)
ball.move(55, 70)
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Accessor Methods

• Weve seen accessor methods in use
• But we havent dclared them or seen their headers
• Heres contains(), from the BBall class
• Instead of void, theres a type now!
• In this case its boolean its what the method
  returns
• Call this the return type
• Result of sending this message to an object will
  be a value or either true or false
• void just means a method has no return type
• So void is a type its just no type

ball.getX() ball.contains(point)
public boolean contains(Location point)
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Accessor Methods

• Heres the entire contains method from BBall
• So what does this method do?
• Says that to test if a BBall contains a point,
  just test if its body (the main filledOval)
  contains the point
• So it tests the body and returns whatever that
  result was
• Something else new here the return statement
• Tells Java what value to return

public boolean contains(Location point) return
body.contains(point)
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Return Statement

• Heres the entire contains method from BBall
• Return statement is used only in non-void methods
• Use it like this
• return expression
• Value produced by expression must be same type as
  methods return type
• See above theyre both boolean

public boolean contains(Location point) return
body.contains(point)
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Return Statement

• What if you wanted to get the size of a BBall?
• Write a method to do it!
• Above methods return type is int
• The expression after the return statement is
  body.getWidth()
• This is also of type int
• This would be an error (and nonsensical!)

public int getSize() return body.getWidth()
public int getSize() return body.getColor()
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Return Statement

• Accessor methods have to have at least one return
  statement
• This is so that the expressions that use calls to
  accessor methods are guaranteed to have a value
• You could have multiple return statements
• Just make sure every possible path through the
  method leads to a return statement

public int getSize() if (isInflated)
return 30 else return 10
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Talking to yourself

• Sometimes we might want an object to send a
  message to itself
• Why?
• Look at move() in BBall
• This has 6 statements to move all the parts of
  the ball

// move the ball by specified offsets public
void move( double dx, double dy) // move
each part of the ball by the offsets provided
body.move(dx,dy) border.move(dx,dy)
vert.move(dx,dy) horiz.move(dx,dy)
leftCut.move(dx,dy) rightCut.move(dx,dy)

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Talking to yourself

• How would you implement moveTo()?
• You could write it with 6 statements, just like
  move()
• One to move each piece of the ball to the new
  location
• Or you could just figure out how far you need to
  move, then call move()

public void moveTo( double x, double y)
//do something
public void moveTo( double x, double y )
// move the ball by the offsets needed to
reach desired location this.move ( x -
body.getX(), y - body.getY() )
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Talking to yourself
public void moveTo( double x, double y )
// move the ball by the offsets needed to
reach desired location this.move ( x -
body.getX(), y - body.getY() )

• What happened there?
• This part computes the difference between the
  point to move to and the balls current location
  (i.e. how far to move)
• This part just calls move() on this ball
• i.e.the same ball that moveTo() was called on
• Tells itself to move the distance we computed

x - body.getX() y - body.getY()
this.move ( x - body.getX(), y -
body.getY() )
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Talking to yourself
public void moveTo( double x, double y )
// move the ball by the offsets needed to
reach desired location this.move ( x -
body.getX(), y - body.getY() )

• this is a special keyword in Java
• If you use this in a method, it refers to the
  object that the method was called on
• If you say
• Then while moveTo() is running, any this inside
  moveTo refers to ball
• Here, the this in moveTo() would refer to
  otherBall

ball.moveTo(5, 6)
otherBall.moveTo(3, 45)
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Talking to yourself
public void moveTo( double x, double y )
// move the ball by the offsets needed to
reach desired location this.move ( x -
body.getX(), y - body.getY() )

• The above version of moveTo() is a lot shorter
  and easier to understand than if we wrote all 6
  statements
• Also, if you added another piece to the ball, you
  wouldnt have to change moveTo()
• Say the ball has a UNC logo on it
• You only have to add an extra move() statement in
  move()
• moveTo will still work as written!

public void move( double dx, double dy)
body.move(dx,dy) border.move(dx,dy)
vert.move(dx,dy) horiz.move(dx,dy)
leftCut.move(dx,dy) rightCut.move(dx,dy)
logo.move(dx, dy)
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Talking to yourself
public void moveTo( double x, double y )
// move the ball by the offsets needed to
reach desired location this.move ( x -
body.getX(), y - body.getY() )

• Its pretty common to talk to yourself in Java
• So much so that Java tries to make it easy for
  you
• If you call a method and leave out the object on
  which to call it, Java will assume you meant this
• You could thus write moveTo like this
• Your choice which to use whichever makes more
  sense to you

public void moveTo( double x, double y )
// move the ball by the offsets needed to
reach desired location move ( x -
body.getX(), y - body.getY() )
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Another example

• Lets look at another custom graphics object
• The T-shirt
• This could be used to soup up your laundry
  program
• You could have shirts that look like shirts!
• Currently, all your clothes are square
• Not exactly a fashion statement, or at least not
  the one you wanted to make

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T-Shirts

• TShirt is kind of like BBall
• It draws an object composed of six subpieces
• Its constructor creates all the subpieces and
  associates names with them
• Its move method moves them all
• Its moveTo method depends on its move method.

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T-Shirts

• There are also a few differences
• TShirts contains method can not just call the
  contains method of one other object
• It has some extra methods (setColor,
  sendToFront).
• You could pretty easily put this in your laundry
  program instead of a FilledRect
• Maybe just change the type

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Custom Classes

• Youve seen move() and moveTo() on rectangles and
  other shapes
• Hopefully this makes it easy to understand the
  idea of defining your own methods in a new class
• You shouldnt think that you can only define
  methods like the existing ones
• You should design methods that are appropriate
  for the role of the object in your program

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Back to the Robot

• Robot might be one object
• made up of FilledRects for the arms, legs, body,
  and head
• Or, he might have custom Leg, Arm, Body, and Head
  objects!
• Each could have methods
• leg.move()
• arm.grab()

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Custom Classes

• You might also want a totally new method on BBall
• In our program, the ball always has to reset to
  the starting point
• How about a reset() method?
• This would put the ball back in its initial
  position

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Adding reset() to BBall

• How would you add reset() to BBall?
• What instance variables would you need?
• How would you change the constructor?
• What would the reset() method look like?

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Adding reset() to BBall

• How would you add reset() to BBall?
• What instance variables would you need?
• How would you change the constructor?
• What would the reset() method look like?

double initX, initY
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Adding reset() to BBall

• How would you add reset() to BBall?
• What instance variables would you need?
• How would you change the constructor?
• What would the reset() method look like?

double initX, initY
initX top initY left
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Adding reset() to BBall

• How would you add reset() to BBall?
• What instance variables would you need?
• How would you change the constructor?
• What would the reset() mutator method look like?

double initX, initY
initX top initY left
public void reset() this.moveTo(initX,
initY)
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Adding reset() to BBall

• Now you can just change onMouseRelease() to call
  reset(), in the class that extends
  WindowController
• Before
• After

public void onMouseRelease(Location
point) if ( hoop.contains(point)
ball.contains(lastMouse) )
score score 2
display.setText("Your score is " score )
ball.moveTo(BALLX,BALLY)

public void onMouseRelease(Location
point) if ( hoop.contains(point)
ball.contains(lastMouse) )
score score 2
display.setText("Your score is " score )
ball.reset()